HID (high intensity discharge) arc lamps are in wide use for general illumination. Applications include roadside street lamps, sports arena illumination, stadium illumination, auto dealership illumination, warehouse illumination, and other purposes requiring a high power of illumination with high efficiency. They tend to be mounted at fairly high elevations requiring maintenance crews to replace.
A typical digital ballast previously used with these lamps is optimized for a fairly narrow range of input line voltages such as from about 100 to about 700 volts AC (alternating current). Line voltages are typically rectified and then boosted to a level of about 400-450 volts DC (direct current) to provide a power source for the last stage of the ballast. When accommodating higher input line voltages above about 300 volts AC the typical electronics become an issue because the rectified and boosted DC voltages become difficult to accommodate.
Other issues with the past ballast design is with variation in initial lamp characteristics and over the life of the ballast. For example, varying lamp impedances either due to initial variations or over lamp life reduce efficiency of some HID lamps. This exacerbates the problem of accommodating varying line input voltages.